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Starting with photosynthesis, autotrophs, such as plants, transform carbon from the atmosphere in the form of carbon dioxide into consumable energy that is ingested by heterotrophs. These organisms can then return the carbon into the atmosphere through respiration or into the soil during decomposition.

Carbon dioxide in the atmosphere is also highly soluble in water so the oceans constitute a major reservoir of carbon.

Carbon dioxide reacts with water to form carbonic acid and then carbonate ions. Calcium carbonate in ocean water is used to build coral skeletons and mollusc shells. Carbon can also be stored for short terms in the bodies of long-lived trees and in the soil or, for a much longer time in the case of fossil fuels, the organic remains of once living organisms.

Finally, carbon stored in sediments and soils of the earth's crust can be melted into magma and vented into the atmosphere during volcanic eruptions, a process called degassing.

27.8: The Carbon Cycle

Carbon is the basis of all organic matter on Earth, and is recycled through the ecosystem in two primary processes: one in which carbon is exchanged among living organisms, and one in which carbon is cycled over long periods of time through fossilized organic remains, weathering of rocks, and volcanic activity. Human activities, including increased agricultural practices and the burning of fossil fuels, has greatly affected the balance of the natural carbon cycle.

Biological Carbon Cycle

All living things are composed of organic molecules that contain atoms of the element carbon. Carbon exists in the atmosphere as carbon dioxide gas, which reacts with water to form bicarbonate. During photosynthesis, primary producers (or autotrophs) convert carbon dioxide and bicarbonate into organic carbon-containing compounds, such as glucose, to provide energy for growth, maintenance and other processes.

Heterotrophs receive organic carbon for growth and maintenance by consuming autotrophs. Through the process of cellular respiration, these organic molecules are broken down to release the energy stored within them. The byproducts of this process are water and carbon dioxide, which is released into the atmosphere through respiration, continuing the cycle.

Carbon can also return to the environment as animal waste or as decaying material from dead organisms. Decomposers, such as bacteria and fungi, break these materials down into carbon dioxide and organic compounds.

Long-Term Geological Cycling

Carbon is stored for prolonged periods of time in the atmosphere, large water bodies, ocean sediment, soil, fossilized animal remains, and the inside of the Earth. Carbon dioxide from the atmosphere becomes dissolved in the ocean’s water, reacts with water molecules and is converted into carbonates. Combined with calcium ions, these carbonates make up the calcium carbonate shells of many marine organisms, such as coral and oysters. When these organisms die, their remains are broken down and may sink to the ocean floor and gradually become part of the sediment. This sediment eventually forms limestone, which constitutes the largest carbon reservoir on Earth.

Large, long-lived trees can also store carbon in their bodies for centuries. As a much longer-term sink (carbon reservoir), the remains of organisms store carbon in the form of fossil fuels—like coal, petroleum and natural gas—deep in the earth over millions of years. Carbon stored deep under the Earth’s surface can return to the surface and the atmosphere during volcanic eruptions.

Human Impacts on the Carbon Cycle

The burning of fossil fuels and wood is releasing substantial amounts of carbon dioxide into the atmosphere, altering the global climate. Additionally, deforestation and increased agricultural practices are affecting the natural carbon cycle by reducing the amount of organic matter in the soil, decreasing carbon storage, and increasing levels of greenhouse gases, like methane, in the atmosphere.

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